In film wrapping, a variety of wrapping methods taking advantage of the film properties are used. For example, many methods are used such as a method of sealing in a bag form, a method of wrapping by twisting a film, a method of wrapping by heat shrinking (to be referred to as a "shrink method" hereinafter), a tight-contact wrap method typified by Saran Wrap.RTM. (made by Asahi Chemical Industry Co., Ltd.), a stretch wrap method and a skin pack method. Each wrapping method is required to have its own wrapping properties, and in a practical sense, film materials, composition, form and properties are selected suitably for each wrapping method.
Of the above methods, the shrink method uses the heat shrinkability of a preliminarily stretched and oriented film. In this method, an article to be wrapped is preliminarily loosely wrapped by a film, for example, an article is wrapped by sealing, and then the film is allowed to shrink by heating it with hot air, infrared rays, hot water or other heating medium to bring the film into tight contact with the content contained therein. The characteristics of this wrapping are that the appearance of the wrapped article is esthetically good, that the commercial value is improved, that the content quality can be visually and tangibly recognized while the article is kept in a hygienic wrapping, that even an irregularly-formed article and even a plurality of articles can be tightly fixed or wrapped in one wrapping, and that the protection performance for the content against vibration and impact is excellent. Further, as compared with a stretch wrapping method which is popular nowadays in supermarkets and will be described later, the above shrink method characteristically permits an increase in wrapping speed. The shrink method also permits the wrapping of an irregularly formed article which cannot be wrapped esthetically by a stretch wrapping and of an article without any container such as a tray. Moreover, the shrink method characteristically permits the wrapping of various articles more tightly. However, on the other hand, the shrink method has defects in that the film is required to be sufficiently heated until the film becomes shrunken. This would result in the likelihood that the sealing portion would break and that the film would deteriorate after being shrunk (strength and optical properties). On the other hand, conventional shrink films have low stretchability, and would break if stretched, to any large extent. Some conventional shrink films cannot simply be stretched due to too strong of a stress caused by the stretching. Further, these films have almost no self-adhesion properties. For these reasons, these shrink films are not at all suitable for stretch wrapping. Conventional shrink films have the above-described defects and problems.
As long as the above defects can be overcome, shrink wrapping is more advantageous than stretch wrapping in use of film area a film, in lesser film thickness and in wrapping speed.
Meanwhile, stretch wrappings have the following excellent characteristics. An easily stretchable film is rich in fitness in which it is free from permanent deformation and the tendency to form a crease when stretched and it can cope with the concavo-convex form and size of an article to be wrapped. A film can be simply fixed by a wrapping tension without loosening by lightly attaching it under pressure or heat-sealing it. A film for fresh foods has proper permeability to gas to prevent the decrease in freshness and weight. The quality of an article can be visually and tangibly known while the article is kept in a hygienic wrapping. The appearance of a package is esthetically good and the commercial value of a wrapped article can be remarkably improved. A wrapping machine can be selected from less expensive manual wrapping machines and high speed automatic wrapping machines as required. Further, an article to be wrapped is not at all heated. As a wrapping method having the above excellent characteristics, the stretch wrapping method is widely used in supermarkets, etc., for wrapping fruits and vegetables, fresh foods, meat and cooked dishes. However, films for the stretch wrapping have the following defects and problems. Due to having low strength, the film is likely to break mechanically during wrapping or in distribution. The film is likely to form holes when sealed. Since the film is imparted with stretchability by taking care not to give too stretch and orientation when produced, the film is characterized with a low modulus of elasticity. The film thickness cannot be extremely decreased as a result of the importance placed on its operational capabilities (e.g. mechanical suitability and manual workability) (in order to avoid deteriorating these capabilities), and accordingly, a film having a large thickness is inevitably used. Many manufacturers have attempted to create films equivalent to or films which are excellent over conventionally widely used films formed of plasticized vinyl chloride (containing 30 to 33% by weight of a plasticizer), and these manufacturers have manufactured a variety of films from various resins by way of trial. However, up to the present time they have not been successful. From the viewpoint of environmental pollution and hygiene, it has been desired to develop a substitute film for the plasticized vinyl chloride film. However, the use of films having inferior qualities and being poor in handling properties is unavoidable at the present time. However, these substitute films being used at present are poor in handling and are used with reluctance in a working site due to their poor wrapping workability. These substitute films are not used in such amounts that they can replace vinyl chloride films. Presently, the amount of these films which are in use is very small.
The shrink wrapping is explained hereinafter, which, of course, does not limit the present invention.
As for shrink wrapping, a stretched film of plasticized polyvinyl chloride (to be referred to as PVC hereinafter) is used in the largest quantity as a high-class shrink wrapping film. This film has superior advantages in that it is capable of heat shrinkage at a high rate and at a relatively low temperature. It permits excellent shrink wrapping in a wide temperature range, and also has a high film elastic modulus (50 to 200 kg/mm.sup.2) and excellent suitability to mechanical wrapping. However, it is inferior in heat sealability and in moisture proof, and it has a hygienic problem due to the presence of a plasticizer and a problem of deterioration with time due to the plasticizer. There are also problems in that it generates a toxic gas such as a chlorine-containing gas when fused with a heating wire for cutting. It also generates a corrosive toxic gas when incinerated after use, and it also becomes stiff and fragile and is likely to break due to having poor resistance to cold when a wrapped article is preserved at a low temperature, handled in a cold area or frozen.
In recent years, a polypropylene-based (to be referred to as PP-based hereinafter) shrink wrap film has been attracting attention. The defect with this film is that the shrinkability is inferior to that of a PVC film. A PP-based stretched film is superior in mechanical properties, moisture resistance and fusion-sealability and excellent as a shrink wrapping film. Further, it has a lower specific gravity than PVC, and is advantageous in terms of material cost.
However, PP is a crystalline polymer having a high softening point, and likely to tear when stretched. Moreover, it has a higher heat-shrink temperature than a conventional stretched film, particularly a PVC film, and shows a small shrinkage at a lower temperature of about 100.degree. C. For this reason, PP is required to be heated to a higher temperature in a shrink wrapping step. The tolerable range of the heating temperature is narrow, and further, the dependence of the shrinkage on temperature is sharp. Therefore, due to a local uneven heating during the wrapping, it shrinks unevenly to cause undesirable defects, such as "creases" and "pockmarks" in the course of practical use. If the pp film is sufficiently heated to prevent the above defect, undesirably, an article to be wrapped would become over-heated, and the PP film would undergo devitrification, form holes by being melted, deteriorates in performance and would result in a large breaking which starts from a sealing portion or an air-discharge opening portion. These constitute serious defects. A PP film also has defects in that a wrapped article is likely to be loose since a stress is released after a certain period of time and the film after wrapping becomes hard and more fragile.
In a conventional polyethylene-based film, the molecule cannot be sufficiently stretched or oriented. Therefore, such a film has a low heat shrinkage, a low heat shrink stress in particular. Further, the film has a high shrink temperature, and it is also inferior in strength and optical properties. The strength of the film binding a wrapped article is also low. Therefore, the film having an increased thickness is used in a special field.
The polyethylene-based film, which is obtained by using high energy rays to cause a sufficient crosslinking reaction on the molecule and by stretching it at a high temperature, has a high heat shrinkage and a high heat shrink stress in a high-temperature range, and such a film is excellent before wrapping over a general polyethylene-based film in optical properties such as transparency and gloss and heat resistance such as melt resistance or resistance to formation of holes. Since, however, it is allowed to shrink in a high-temperature range, its performance greatly deteriorates as compared with a film used for shrink wrapping at a low-temperature (particularly, the optical properties greatly decrease). Furthermore, the above film has a defect in that the wrapping speed is inferior for the following reasons. It has heat shrink properties in which it rapidly shrinks under heat. It is difficult to heat-seal due to high crosslinking. It is easily broken due to inferior resistance to tearing, and difficult to cut with a heating wire. As described above, to maintain the performance of a film, one of the important features required of shrink wrapping is that the wrapping can be carried out effectively at a low temperature, and this feature is also required when an article to be wrapped is brought into contact with the film (particularly when fresh foods are wrapped).
In case that a known film which has a high shrink temperature (it is practically required to allow the film to shrink at least 20% in the width and length directions), or which shrinks rapidly under heat (due to high shrink dependency upon temperature) is used wrapping is required to be conducted at a temperature greatly exceeding the melting point of the polymer and under very narrow conditions in order to particularly improve the finish of a wrapped article. This case involves a contradicting problem in that the properties of the film are greatly decreased.
On the other hand, when a stretched film is produced from PP as a material, there is employed a method in which PP is melted in an extruder, extruded through dies and rapidly cooled to obtain a film (hereinafter referred to as "raw film") in a tubular form, the tubular raw film is reheated at a high temperature of 130.degree. to 150.degree. C., and stretched by introducing air into the tube. When the material is a low-density polyethylene, it is considered that biaxial stretching for high stretching and orienting easily causes breaking at a processing time and that the stretching is technically very difficult.
For this reason, it is a general practice to employ a one-stage inflation method in which the material is extruded, e.g., at a temperature of 180.degree. to 220.degree. C. and then immediately inflated while properly cooling it with air to form a film having a predetermined size.
The above method has a characteristic feature in that the film can be produced at a very low cost with ease. However, intermolecular flow is likely to occur, and no satisfactory molecular orientation can be obtained by stretching. Further, the film shows very poor optical properties. Therefore, the heat shrinkage and heat shrink stress are small, and the heat shrink temperature region is on a high-temperature side. Only when the film has an increased thickness, can it be used in a special field. For this reason, it is conceivable to prevent the intermolecular flow and obtain a sufficient molecular orientation by employing a method which comprises forming a low-density polyethylene, irradiating the formed product with high-energy radiation under proper conditions to cause a partial crosslinking reaction, reheating the product up to a temperature beyond the melting point (e.g., 140.degree. C.) and stretching it. In this case, however, the degree of the shrinkage is low at a lower temperature, and the resultant film is likely to tear.
For other new wrapping film, a diversity of composite, multi-layered films are known.
With a technical advance in required properties, an increasing number of composite films tend to be produced. For example, there is a composite film obtained by melt-laminating other resin on a nearly unstretched film or a stretched film. Specifically, commercially available is a film having improved heat-sealability obtained by melt-laminating other resin on an unstretched polypropylene prepared by a casting method (called CPP) or an oriented polypropylene (OPP) or a film which is coated with vinylidene chloride-based latex to impart it with barrier properties (called K coat film). A variety of films or a combination of these films are selected depending upon use.
On the other hand, generally known is a co-extruded film obtained by melting a plurality of resins in separate extruders, extruding the resins through a multi-layer die while converging and fusing them inside the die and cooling the resultant product to form a film or sheet.
However, when a film having the above multi-layer structure of which at least one layer is highly stretched and oriented is obtained from a combination of resins having mutually different properties (melting point, softening point, melt index and orientation properties), the optimum extrusion conditions and stretching conditions differ depending upon the resins. Therefore, when such a film is produced by a prior art method, many problems occur including defective phenomena such as a nonuniform section, streaks including an insufficiently stretched portion, a puncture, breach, peeling of layers and whitening due to interfacial toughening. Further, there is obtained a film having different properties from those of an intended film. Thus, these defects have not yet been overcome. In order to overcome these defects, the present inventors completed composite films, which are disclosed in JP,A 55-118859 (corresponding to U.S. Pat. No. 4,430,378) and JP,A 58-175635. However, these films have not as yet sufficiently met the above requirement level. The present invention has achieved the creation of a film which fully satisfies the properties lacking in these composite films, which widens the use field, and which is capable of exhibiting high performances even though it has a much smaller thickness than any conventional film and is excellent in cost performance. These features will be more clear in comparison made with the Comparative Examples which will follow later.
The present status of the stretch wrapping, a second field to which the film of the present invention is directed, will be described hereinafter.
In this field, films formed from a soft PVC containing, as a material, a large amount, e.g., as much as about 30% by weight (nearly 50 vol %) of a plasticizer are mainly commercially used. If a large amount of a plasticizer is not incorporated, it would be difficult to process these films, or it would be impossible to impart the film with flexible properties, and these films could not be used in this field. Since a large amount of a plasticizer, such as DOP, DOA, etc., is incorporated, these films would have the problems in that the amount of permeating water vapor would increase, and a wrapped article would likely be altered, that these plasticizers would likely migrate to a wrapped article thus contaminating the article, that a gas of the plasticizer and a corrosive chlorine gas would be generated when the film is fused during a wrapping work, which would be hygienically undesirable, that a toxic gas would be generated when a used film is incinerated, and that the film would become less flexible and fragile and would be likely to break due to its poor resistance to cold when a wrapped article is preserved at a low temperature, which is as already described.
Meanwhile, single-layered films formed of a high-density polyethylene, a low-density polyethylene and a PP-based polymer among the generally usable polyolefins have excellent properties over the above defects. However, these films do not have the other important properties required for use in the field to which the present invention is directed to. It has not been possible to form a practically usable stretch wrapping film which satisfies all of the following characteristics as a wrapping film. That is, any film for use in stretch wrapping is required to satisfy the following properties simultaneously.
a. The film is to be excellent in film-film adhesion. PA1 b. The film is to have a proper degree of (excellent) recoverability from deformation, a proper degree of elastic elongation and high mechanical strength. PA1 c. The film is to have a proper degree of lubricity. PA1 d. The film is to be excellent in optical properties such as transparency and gloss. PA1 e. The film is to a proper degree of gas permeability. PA1 f. The film is to retain no water drops and to be excellent in anti-fogging property. PA1 g. The film is to be excellent in wrapping workability. PA1 h. The film is to have heat resistance sufficient to endure heat in sealing. PA1 said (A) being at least one copolymer selected from low-density polyethylenes, copolymers of ethylene with at least one monomer selected from vinyl ester monomers, aliphatic unsaturated monocarboxylic acids and alkylesters of said monocarboxylic acids, and derivatives thereof, PA1 said (B) being at least one soft thermoplastic elastomer having a Vicat softening point of 80.degree. C. or lower, PA1 said (C) being selected from a crystalline polypropylene, a crystalline polybutene-1 and a crystalline poly-4-methylpentene-1 or a mixture thereof. PA1 said (A) being at least one copolymer selected from low-density polyethylenes, copolymers of ethylene with at least one monomer selected from vinyl ester monomers, aliphatic unsaturated monocarboxylic acids and alkylesters of said monocarboxylic acids and derivatives thereof. PA1 said (B) being a soft thermoplastic elastomer having a Vicat softening point of 80.degree. C. or lower, PA1 said (C) being any one of a crystalline polypropylene, a crystalline polybutene-1 and a crystalline poly-4-methylpentene-1 or a mixture thereof, extruding the resins through a multi-layer die, rapidly cooling and solidifying the extrudate by means of a liquid cooling medium to prepare a tubular or flat master roll, optionally heating it to a temperature of not more than a temperature of 120.degree. C. and stretching it at a stretching temperature in the range of 30.degree. to 110.degree. C., and at an area stretch ratio of 4 times to 30 times. Preferred embodiments thereof are as follows. PA1 (1) 0.05.ltoreq.B/(A+B).ltoreq.0.90, PA1 (2) 0.30.ltoreq.B/(B+C).ltoreq.0.90 or PA1 (3) 0.05.ltoreq.B/(A+B).ltoreq.0.90 and 0.05.ltoreq.C/(A+B).ltoreq.2.0, PA1 (1) 0.07.ltoreq.B/(A+B).ltoreq.0.70 PA1 (2) 0.40.ltoreq.B/(B+C).ltoreq.0.87 or PA1 (3) 0.07.ltoreq.B/(A+B).ltoreq.0.70 and 0.07.ltoreq.C/(A+B).ltoreq.1.0, PA1 (1) 0.10.ltoreq.B/(A+B).ltoreq.0.50 PA1 (2) 0.50.ltoreq.B/(B+C).ltoreq.0.85 or PA1 (3) 0.10.ltoreq.B/(A+B).ltoreq.0.50 and 0.10.ltoreq.C/(A+B).ltoreq.1.0. PA1 (1) Five layers PA1 (2) Six layers PA1 (3) Seven layers PA1 (4) Eight layers PA1 S/R/H/SBC/R/SBC/H/S, PA1 (5) Double
For example, when an unstretched film of PP is stretched, it causes a phenomenon called necking, that is, it is locally stretched and the thickness becomes extremely nonuniform. Even after a load is removed, the stretched portion remains as is. Therefore, the appearance of a wrapped article is extraordinarily impaired, and the purpose in wrapping is not achieved. A stretched film of the same is hard, strong and little extendable so that a very large force is required to extend it, and an article being wrapped would be destroyed. This film is also without any adhesion properties, and it is therefore required to incorporate as much as 5% by weight of a plasticizer such as low-molecular-weight polybutene in order to impart it with stretchability and adhesion properties. In this case, however, since polyolefin is, unlike PVC, not at all capable of retaining the plasticizer, most of the plasticizer bleeds out on the surface and makes the surface sticky, and such a film is not suitable for practical use.
A high-density polyethylene is also too hard to simply stretch. Even if stretched forcibly, it causes a nonuniform thickness due to necking, and results in a similar result. Further, it is all opaque or without gloss and cannot be put to practical use.
Films formed from low-density polyethylenes are much softer than the above film. However, unstretched films of this type cause a necking phenomenon in packaging, and they have little recoverability from deformation, low strength, low transparency, film-film adhesion property poor and a problem in practical use. Therefore, they cannot meet the object of the present invention. Further, when these films are sufficiently stretched (e.g., at an area stretch ratio of 20 times) at a temperature equal to or higher than the melting point according to a conventional method after imparted with a crosslinked structure by electron beam and rendered easily stretchable, the resultant films have the same defects as those of PP, and they cannot be used as a substrate for stretch packaging which is an object of the present invention.
A film formed from an elastic elastomer having nearly complete recoverability from deformation such as a styrene-butadiene copolymer and a rubber substrate formed from other material is free from a phenomenon of said necking. However, such a film has problems concerning optical properties and hygienic storage of food. Besides these, the stress strength of elongation is nearly completely in proportion to the degree of elongation, and the response in recovery from deformation instantaneously occurs without any delay. There is therefore a problem in that the film takes its original shape immediately before the end portion of the film is set under an article to be wrapped or a tray. Thus, such a film has properties due to which it can be hardly employed for the use of the present invention.
Of these polyolefin-based films, those that are produced by mixing a main component such as linear low-density polyethylene, particularly an ultra-low-density polyethylene, crystalline 1,2-polybutadiene and an ethylene-vinyl acetate copolymer (EVA) with an anti-fogging agent and a tackifier and by forming the resultant mixtures into films by conventional methods (T-die method and air-cooling inflation method) are on trial in the market. However, these films have many of the defects as described above, and presently have not as yet arrived at the level of replacing conventional films.
These films cannot satisfy all of properties which are mutually contradicting such as easiness in elongation in wrapping, heat resistance of a sealing portion, easiness in sealing and prevention of breach in wrapping caused by insufficient film strength, and have unsatisfactory properties. For example, in an EVA film, it is required to increase the content of vinyl acetate (VAc) in EVA in order to make the film easily elongated. In this case, however, the bottom portion (under the tray) tends to be melted under heat and becomes broken in sealing. To prevent this, it is required to increase the film thickness from 16 .mu.m up to 20.mu., 22.mu. or 24.mu.. When the film thickness is increased, however, the defects of the film also increases. That is, it is difficult to elongate the film, the film hardly adheres to the film before sealing, and the film is likely to break due to its decreased strength. Further, the film cost disadvantageously increases.
When other polymers, particularly a low-density polyethylene (particularly a linear type one) or PP-based rubber is incorporated, another problems arise. That is, the important properties such as transparency and gloss tend to be decreased.